Provided by: manpages-dev_5.02-1_all bug

NAME

       setns - reassociate thread with a namespace

SYNOPSIS

       #define _GNU_SOURCE             /* See feature_test_macros(7) */
       #include <sched.h>

       int setns(int fd, int nstype);

DESCRIPTION

       Given a file descriptor referring to a namespace, reassociate the calling thread with that
       namespace.

       The fd argument is a file descriptor referring to  one  of  the  namespace  entries  in  a
       /proc/[pid]/ns/  directory;  see namespaces(7) for further information on /proc/[pid]/ns/.
       The calling thread will be reassociated with the corresponding namespace, subject  to  any
       constraints imposed by the nstype argument.

       The  nstype  argument  specifies  which  type  of  namespace  the  calling  thread  may be
       reassociated with.  This argument can have one of the following values:

       0      Allow any type of namespace to be joined.

       CLONE_NEWCGROUP (since Linux 4.6)
              fd must refer to a cgroup namespace.

       CLONE_NEWIPC (since Linux 3.0)
              fd must refer to an IPC namespace.

       CLONE_NEWNET (since Linux 3.0)
              fd must refer to a network namespace.

       CLONE_NEWNS (since Linux 3.8)
              fd must refer to a mount namespace.

       CLONE_NEWPID (since Linux 3.8)
              fd must refer to a descendant PID namespace.

       CLONE_NEWUSER (since Linux 3.8)
              fd must refer to a user namespace.

       CLONE_NEWUTS (since Linux 3.0)
              fd must refer to a UTS namespace.

       Specifying nstype as 0 suffices if the caller knows  (or  does  not  care)  what  type  of
       namespace  is  referred  to by fd.  Specifying a nonzero value for nstype is useful if the
       caller does not know what type of namespace is referred to by fd and wants to ensure  that
       the  namespace  is  of  a  particular  type.   (The  caller might not know the type of the
       namespace referred to by fd if the file descriptor was opened by another process and,  for
       example, passed to the caller via a UNIX domain socket.)

   Details for specific namespace types
       Note  the  following  details  and restrictions when reassociating with specific namespace
       types:

       User namespaces
              A process reassociating itself with a user namespace must  have  the  CAP_SYS_ADMIN
              capability in the target user namespace.  (This necessarily implies that it is only
              possible to join a descendant user namespace.)  Upon successfully  joining  a  user
              namespace,  a  process is granted all capabilities in that namespace, regardless of
              its user and group IDs.

              A multithreaded process may not change user namespace with setns().

              It is not permitted to use setns() to reenter the caller's current user  namespace.
              This  prevents  a  caller  that  has  dropped  capabilities  from  regaining  those
              capabilities via a call to setns().

              For security reasons, a process can't join a new user namespace if  it  is  sharing
              filesystem-related  attributes  (the  attributes whose sharing is controlled by the
              clone(2) CLONE_FS flag) with another process.

              For further details on user namespaces, see user_namespaces(7).

       Mount namespaces
              Changing the mount namespace requires that the caller possess  both  CAP_SYS_CHROOT
              and  CAP_SYS_ADMIN  capabilities in its own user namespace and CAP_SYS_ADMIN in the
              user namespace that owns the target mount namespace.

              A  process  may  not  be  reassociated  with  a  new  mount  namespace  if  it   is
              multithreaded.

              See  user_namespaces(7) for details on the interaction of user namespaces and mount
              namespaces.

       PID namespaces
              In order to reassociate itself with a new PID namespace, the caller must  have  the
              CAP_SYS_ADMIN  capability  both in its own user namespace and in the user namespace
              that owns the target PID namespace.

              If fd refers to a PID namespace, the semantics are somewhat  different  from  other
              namespace types: reassociating the calling thread with a PID namespace changes only
              the PID namespace that subsequently created child processes of the caller  will  be
              placed in; it does not change the PID namespace of the caller itself.

              Reassociating  with  a PID namespace is allowed only if the PID namespace specified
              by fd is a descendant (child, grandchild,  etc.)   of  the  PID  namespace  of  the
              caller.

              For further details on PID namespaces, see pid_namespaces(7).

       Cgroup namespaces
              In  order  to  reassociate itself with a new cgroup namespace, the caller must have
              the CAP_SYS_ADMIN capability both in  its  own  user  namespace  and  in  the  user
              namespace that owns the target cgroup namespace.

              Using  setns() to change the caller's cgroup namespace does not change the caller's
              cgroup memberships.

       Network, IPC, and UTS namespaces
              In order to reassociate itself with a new  network,  IPC,  or  UTS  namespace,  the
              caller must have the CAP_SYS_ADMIN capability both in its own user namespace and in
              the user namespace that owns the target namespace.

RETURN VALUE

       On success, setns() returns 0.  On failure, -1 is returned and errno is  set  to  indicate
       the error.

ERRORS

       EBADF  fd is not a valid file descriptor.

       EINVAL fd refers to a namespace whose type does not match that specified in nstype.

       EINVAL There is problem with reassociating the thread with the specified namespace.

       EINVAL The  caller  tried  to  join  an  ancestor  (parent,  grandparent,  and  so on) PID
              namespace.

       EINVAL The caller attempted to join the user namespace in which it is already a member.

       EINVAL The caller shares filesystem (CLONE_FS) state (in particular, the  root  directory)
              with other processes and tried to join a new user namespace.

       EINVAL The caller is multithreaded and tried to join a new user namespace.

       ENOMEM Cannot allocate sufficient memory to change the specified namespace.

       EPERM  The calling thread did not have the required capability for this operation.

VERSIONS

       The  setns()  system call first appeared in Linux in kernel 3.0; library support was added
       to glibc in version 2.14.

CONFORMING TO

       The setns() system call is Linux-specific.

NOTES

       Not all of the attributes that can be shared when a new thread is created  using  clone(2)
       can be changed using setns().

EXAMPLE

       The  program below takes two or more arguments.  The first argument specifies the pathname
       of a namespace file in an existing /proc/[pid]/ns/  directory.   The  remaining  arguments
       specify  a  command  and  its arguments.  The program opens the namespace file, joins that
       namespace using setns(), and executes the specified command inside that namespace.

       The following shell session demonstrates the use of this program  (compiled  as  a  binary
       named  ns_exec)  in  conjunction with the CLONE_NEWUTS example program in the clone(2) man
       page (complied as a binary named newuts).

       We begin by executing the example program in clone(2) in  the  background.   That  program
       creates  a  child  in  a  separate  UTS  namespace.  The child changes the hostname in its
       namespace, and then both processes display the hostnames in their UTS namespaces, so  that
       we can see that they are different.

           $ su                   # Need privilege for namespace operations
           Password:
           # ./newuts bizarro &
           [1] 3549
           clone() returned 3550
           uts.nodename in child:  bizarro
           uts.nodename in parent: antero
           # uname -n             # Verify hostname in the shell
           antero

       We  then  run the program shown below, using it to execute a shell.  Inside that shell, we
       verify that the hostname is the one set by the child created by the first program:

           # ./ns_exec /proc/3550/ns/uts /bin/bash
           # uname -n             # Executed in shell started by ns_exec
           bizarro

   Program source
       #define _GNU_SOURCE
       #include <fcntl.h>
       #include <sched.h>
       #include <unistd.h>
       #include <stdlib.h>
       #include <stdio.h>

       #define errExit(msg)    do { perror(msg); exit(EXIT_FAILURE); \
                               } while (0)

       int
       main(int argc, char *argv[])
       {
           int fd;

           if (argc < 3) {
               fprintf(stderr, "%s /proc/PID/ns/FILE cmd args...\n", argv[0]);
               exit(EXIT_FAILURE);
           }

           fd = open(argv[1], O_RDONLY); /* Get file descriptor for namespace */
           if (fd == -1)
               errExit("open");

           if (setns(fd, 0) == -1)       /* Join that namespace */
               errExit("setns");

           execvp(argv[2], &argv[2]);    /* Execute a command in namespace */
           errExit("execvp");
       }

SEE ALSO

       nsenter(1), clone(2), fork(2), unshare(2), vfork(2), namespaces(7), unix(7)

COLOPHON

       This page is part of release 5.02 of the Linux man-pages project.  A  description  of  the
       project,  information  about  reporting  bugs, and the latest version of this page, can be
       found at https://www.kernel.org/doc/man-pages/.